High Performance Fibers (HPFs) are being proposed for expanding uses in many specialized applications, such as military and aerospace (turbo machinery, rockets, advanced structures), automobile, biomedical, energy, and other applications that require advanced materials with exceptional strength, stiffness, heat resistance, and/or chemical resistance. HPFs are sought when a combination of extreme material properties is required which cannot be met by existing metal filaments or by carbon, glass, vegetal or mineral fibers, or by synthetic polymer fibers. HPF composite systems generally include a plurality of coated fibers, distributed within a “matrix.” When the fibers are ceramic and the matrix is a ceramic, the resultant composite structure is usually referred to as a Ceramic Matrix Composite or CMC. When the matrix is a metal, the resultant composite structure is called a Metal Matrix Composite or MMC. When the matrix is polymer-derived the composite is a Polymer Matrix Composite or PMC.
Composites derive their gross mechanical properties from the properties of the fiber and matrix constituents. In particular, the fiber properties most highly valued by designers of MMCs and CMCs are: stiffness as measured by Young's Modulus; tensile strength as measured by loading fibers to tensile failure; and creep resistance or resistance to “stretching” during high temperature loading.
Existing commercial inorganic fibers are produced via a process that forces a liquid polymeric precursor containing atoms of interest (along with other elements that together create the liquid) through a spinneret, which is a structure reminiscent of a shower head with a plurality of holes through which the precursor liquid is extruded. The volatile chemical species are flashed off in this process, thereby producing “green” fiber which is then transported to a furnace or kiln to attempt to drive off the remaining unwanted elements. These elements can only asymptotically be driven off, which means that unwanted elements remain in the fiber, thereby affecting fiber properties, in particular survivability at high temperatures, but also fiber microstructure which affects other properties such as stiffness, tensile strength and creep resistance.
Fibers produced via the spinneret process typically exhibit islands of generally equiaxed crystalline material surrounded by amorphous material which includes unwanted elements. The crystals are generally equiaxed in that crystal grains show no particular elongation or aspect ratio, and are randomly oriented and distributed throughout the fiber. This combination of crystal and amorphous material gives rise to a set of stiffness, tensile strength and creep resistance properties associated with the process, post-processes and the fiber material system of interest (eg silicon carbide, boron carbide, tungsten carbide, etc.), all of which can be improved upon by the present invention.